Field of the Invention
The invention relates to a databus including n+1 (n.gtoreq.2) lines which form n true-only lines and a monitoring line (single complement) and lead from n input blocks to n output blocks.
Databuses are often formed, for structural reasons, from "true/complement lines" in which two lines are required for transmitting a data bit between an input block and an output block. Consequently, if required, the data can be further processed differentially after the output block.
In addition, such a configuration of two lines for transmission of a data bit makes it possible to specify for each data bit when that bit on the bus is or is not valid. Specifically, for reliable data transmission, it is important to know when the respective data on the bus are valid, and when they are not.
In order to carry out such a validity check it is possible, for example, to use an unused state of the two mutually complementary data lines in order to indicate that there are no valid data on the bus. That can be done, for example, by assigning a "high" state to the two complementary data lines to indicate that unused state. If one of the two lines then assumes the other, "low" level, it can be deduced therefrom that a valid data bit is present.
In other words, if both lines are at the "high" state, that means that there are no valid data on that line of the bus. However, if the two lines are at different levels, specifically "high" and "low", then it is possible to deduce therefrom that a valid data bit is intended to be transmitted over those lines.
It is, of course, also possible to reverse the potentials, that is to say, for example, to provide a "low" state in order to indicate that there are no valid data on the respective lines.
However, since p-channel MOS transistors produce less current, charging is preferably carried out in the precharging phase, when time is not critical and discharging again is carried out in the switching phase, when time is critical, through n-channel MOS transistors, which are faster than p-channel MOS transistors. The "high" state is thus preferred in order to indicate that there are no data on the respective lines of the bus.
In summary, it can thus be stated that two lines of a databus should be provided, per se, in each case for transmission of a data bit. If the two lines are at the same state, that indicates that a transmitted data bit is invalid, while if the two lines are at different states, that indicates that the transmitted data bit can be regarded as being valid.
In the case of relatively long wiring having a bus between two blocks with two lines for one data bit in each case, a large area is required so that such a procedure does not make sense, for that reason. However, if only one line is provided per data bit, then the information is lacking as to when a respective data bit can or cannot be regarded as being valid.
For that reason, the solution until now has been to provide only one line per bit ("true only") and to produce a monitoring signal in addition. That has the advantage of requiring only n+1 lines instead of two n lines, to transmit n data bits. It is thus important how such a monitoring signal is produced. Specifically, if it is derived from another auxiliary signal, then it is necessary to ensure that the data bits are valid before the monitoring signal, and that can be achieved by the use of delay elements. However, such a procedure necessarily leads to a time window, so that the data transmission becomes longer overall.